REDUCTION WITHOUT TETRAD-FORMATION 263 



pause following the first division; and he believes this to be the 

 same division as that seen during the anaphase. Carnoy and Le Hrun 

 ('99) reach the same result in the formation of the polar bodies in 

 Triton, though their general account of the heterotypical mitosis 

 differs very considerably from that of other authors, the rings being 

 stated to arise by a double instead of a single longitudinal split 

 These observers describe the rings of the early anaphase as having 

 almost exactly the same double cross-form as those in Thahisscma ox 

 Ztrphcea (Griffin, '99), but believe them to arise in a manner nearly 

 in accordance with Strasburger's abandoned view of 1895,1 and with 

 Guignard's ('98, 2) and Gregoire's ('99) latest results on the flowering 

 plants, the ring being stated to arise by a double longitudinal split""- 

 ting, as explained at page 265. 



In the elasmobranch Scy Ilium Moore ('95) finds twelve (the re- 

 duced number) ring-shaped chromosomes at the first division. These 

 closely resemble tetrads ; but a resting stage follows, and the second 

 division is likewise stated to be of the heterotypical form, l^oth divi- 

 sions are stated to be equation-divisions — a conclusion well sup- 

 ported in case of the first, but so far from clear in the .second that a 

 careful reexamination of the matter is highly desirable. 



In mammals the first division is of the heterotypical form (Her- 

 mann, '89, Lenhossek, '98), though the rings are much smaller than 

 ni the salamander, recalling those seen in arthropods. No true 

 tetrads are, however, formed, and the two divisions are separated by 

 a resting period. The character of the second division is undeter- 

 mined, though Lenhossek believes it to be heterotypical, like the first. 



(b) Plants. — It is in the flowering plants, where reduction likewise 

 occurs, as a rule, without true tetrad-formation, that the contradiction 

 of results reaches its cHmax ; and it must be said that until further 

 research clears up the present confusion no definite result can be 

 stated. The earlier work of Strasburger and Guignard indicated that 

 no reducing division occurred, the numerical reduction being directly 

 effected by a segmentation of the spireme-thread into half the somatic 

 number of chromosomes. Thus these observers found in the male 

 that the chromosomes suddenly appeared in the reduced number 

 (twelve in the lily, eight in the onion) at the first division of the 

 pollen-mother-cell, and in the female at the first division of the 

 mother-cell of the embryo-sac. The subsequent ])henomena differ 

 in a very interesting way from those in animals, owing to the fact 

 that the two maturation-divisions are followed in the female by one 

 and in the male by two or more additional divisions, in both of which 

 the reduced number of chromosomes persists. In the male the two 

 maturation-divisions give rise to four pollen-grains, in the female to 



1 Cf. p. 269. 



